1. Field of the Invention
The present invention relates to a video compression system, and more particularly, to device and method for converting a frame rate, in which a motion compensated frame is newly produced and applied to a required place for converting a frame rate.
2. Description of the Related Art
Up to now, application of the digital video compression has been restricted to communication media and storage media due to a capacity limitation of the media. However, as the MPEG-2 in which standardizations directed to a high quality broadcasting are push forwarded is employed as a new broadcasting system since it can realize a high picture quality and a high compression ratio, the MPEG-2 (Motion Picture Expert Group) became a motive power for exploring a new era of broadcasting. In connection with this, with a device employing an MPEG-2 decoder are applied to TV, until now, a broadcasting signal is merely digitized, compressed, and transmitted/received through a satellite, cable, or as a terrestrial wave, without deviating the broadcasting signal itself from the present TV system of NTSC, PAL, or SECAM. In other words, without converting the broadcasting signal itself, the MPEG-2 decoder converts a frame rate of the digitized, compressed and encoded signal of a motion picture so as to be in conformity with an existing TV broadcasting signal. For example, in the case of an NTSC system, the present TV broadcasting system has a frame rate of 30 frames per a second, and in the case of a PAL system, the present TV broadcasting system has a frame rate of 25 frames per a second, regardless of a frame rate of an incoming source signal, for being displayed on a screen.
FIG. 1 illustrates a block diagram of a background art device for decoding an MPEG-2 video signal, provided with a VLD(Variable Length Decoding) part 11 for receiving and subjecting an MPEG-2 video bitstream to variable length decoding to provide original serial data of a zigzag scanned 8.times.8 matrix, an inverse scan part 12 for converting the serial data from the VLD part 11 into a two dimensional data array of 8.times.8 matrix, an inverse quantization part 13 for using a Q.sub.-- table in subjecting the two dimensional data array of 8.times.8 matrix from the inverse scan part 12 to inverse quantization, an IDCT(Inverse Discrete Cosine Transform) part 14 for subjecting the inverse quantized data in the inverse quantization part 13 to inverse DCT, a memory part 15 for storage of prior pictures, a motion compensating part 16 for using the prior picture stored in the memory 15 and a motion vector of a presently received picture in reproducing a picture from the IDCT part 14 into original extended series of pictures if the picture from the IDCT part 14 is either a B(Bidirectional) picture or a P (Predictive) picture, and a display controlling part 17 for presenting a motion compensated data after rearrangement or as they are depending on a picture type. Being a value used in determination of a quantizing step in an encoding in a transmitter side, the Q.sub.-- table provided to the inverse quantization part 13 comes from the transmitter side.
FIG. 2 illustrates a regular frame rate by an MPEG-2 video decoder processing in a case when a number of B pictures is two in FIG. 1.
In the aforementioned background art device for decoding an MPEG video signal, upon reception of an encoded MPEG-2 video bitstream from the transmitter side, the VLD part 11 turns the encoded MPEG-2 video bitstream back to original horizontal/vertical frequency bands. That is, the received MPEG-2 video bitstream, being variable length encoded by zigzag scanning, exists in series. Accordingly, the VLD part 11 turns the variable length encoded values by zigzag scanning back to original one dimensional DCT coefficients. The one dimensional DCT coefficients turned back to original state is provided to the inverse scan part 12 and converted into DCT coefficients of a 8.times.8 matrix. The inverse quantization part 13 uses a Q-table used in the encoding in subjecting block units of DCT coefficients converted into a two dimensional data array of 8.times.8 matrix received from the inverse scan part 12 to inverse quantizing and provides to the IDCT part 14. Having its energy components compressed already to be concentrated on a low frequency side, with almost all high frequency components converted into 0, values provided to the IDCT part 14 are restored through the IDCT part 14. If data restored through the IDCT part 14 is an I picture, the I picture is a perfect one that can be displayed as it is, and data restored through the IDCT part 14 is a B or P picture, the picture is an imperfect picture that can be displayed through the motion compensating part 16. That is, with reference to the I picture, a motion vector, information representing a motion, may be considered as "0", which need not any motion compensation, and B or P picture can be motion compensated and restored to an original picture using a prior picture stored in the memory part 15. The motion vector MV is a two dimensional vector indicating an offset of a coordinate of a field in a prior frame from a coordinate of the present picture or field for use in a motion compensation. For an motion prediction in an encoder, first of all, the motion vector MV should be obtained. Because one macroblock may have four motion vectors at the maximum, of which bit amount is too voluminous to pass as they are, a difference of motion vectors of the present macroblock from a right prior macroblock is variable length encoded for transmission. The motion compensating part 16, provided for restoring B or P picture obtained through a prediction, uses a prior picture stored in the memory part 15 and a motion vector on the present B or P picture from the IDCT 14 in making an one directional or bidirectional prediction of the B or P picture for reproducing a perfect image of the B or P picture. If a number of the B pictures is two, the MPEG-2 picture may have a sequence of IBBPBBP - - - , of which B pictures can be decoded only when P or I picture which comes later in terms of time should be used. It implies that a decoding should be carried out in a sequence of IPBBPBB - - - while a display should be carried out in another sequence of IBBPBBP - - - . Accordingly, the display controlling part 17 presents pictures either after rearranging the pictures or as they are depending on a picture type. If the number of the B pictures is two, a series of frames provided from the display controlling part 17, being I, B1, B2, P, - - - as shown in FIG. 2, are displayed on the screen with a frame rate of F25(PAL) or F30(NTSC) when the frame rate is regular. That is, the NTSC system has a frame rate of 30 frames per a second and the PAL system has a frame rate of 25 frames per a second. Accordingly, a film rate(24 frames per a second) in which a signal is processed in a film mode of a digital TV standard(MPEG-2) should be converted into an existing NTSC or PAL system. To do this, if a video signal is received in the film rate, the video signal is inserted appropriately and repeatedly into a whole video sequence in field units, so that the whole video sequence has an intended frame rate. In this instance, the MPEG video signal thus obtained by repetition has a separate syntax which can indicate to the MPEG-2 decoder for easy display of the video signal in conformity with the repeated signal. That is, if a frame rate conversion indicative signal first.sub.-- field.sub.-- repeat provided to the display controlling part 17 is enabled, the display controlling part 17 controls a video signal received at the present time to be displayed repeatedly.
FIG. 3 illustrates a frame rate conversion of digital video signal from 24 to 30.
Each of received frames F1, F2, F3, F4, - - - , F24 is divided into a top field f1 and a bottom field f2, and a new decoded frame F1, F2, F3', F4', - - - , F30 is produced from the divided top fields f1 and the bottom fields f2, i.e., a new frame is produced from adjacent two frames which are the most closely related. That is, a new decoded frame F3' is produced from the top field f1 of the frame F2 and the bottom field f2 of the frame F3, i.e., ##EQU1## and another new frame F4' is produced from the top field f1 of the frame F3 and the bottom field f2 of the next frame F4. By repeating production of new frames from the most closely related adjacent frames, the frame rate conversion from 24 to 30 is done.
However, the aforementioned frame rate conversion of alternating repetition of fields has a problem of blurring of motion components in which motion portions of an image become not clear caused by mere repetition of the fields.
And, of the flicker of pictures due to 25 or 30 frame rate display of one image of the MPEG-2 decoder, though the flicker in the case of NTSC system of 30 frame rate causes no significant problem, the flicker in the case of
system of 25 frame rate causes a significant problem.
Also, in the case of slow motion, the mere repetitive memorization and display of fields or frames results in the slow motion not smooth, with a requirement for an additional memory.